It is often necessary that modern solid state equipment include protection circuitry for preventing the destruction of transistors in the event that either the temperature of the protected transistor exceeds a predetermined maximum value, the current magnitude of the protected transistor increases above a maximum value or the voltages and currents of the protected transistor approach the limits of the "safe area." More specifically, present day solid state voltage regulators, current regulators and audio power amplifiers often include a "series" sense resistor connected in series between a protected transistor and the output terminal of the circuit. Threshold sensitive circuitry is coupled between the sense resistor and the driving circuitry of the protected transistor. When the voltage across the sense resistor reaches a predetermined magnitude, the threshold circuit is rendered operative and reduces the base drive to the output transistor so that the current thereof is reduced before the protected transistor is damaged. This protection scheme has been employed both in discrete and integrated circuits.
The foregoing prior art current controlling circuit has several disadvantages. More specifically, in integrated circuit applications the sense resistor is usually fabricated either near to or as part of the protected transistor. Such series sense resistor usually conducts the entire output current. Since the sense resistor constantly creates heat, the protected transistor is made larger than it otherwise would have to be so that it can dissipate the heat transferred thereto from the sense resistor. Moreover, the protected transistor is made larger to provide the necessary drive power for the series sense resistor which forms part of the load thereof. The resulting increase in die surface area of the protected transistors, which are likely to be the largest components of the chip, results in increased cost, particularly in monolithic integrated circuits utilizing this technique. The increase in die area also results in decreased reliability.
Voltage regulators are often required to have no more than a maximum input-output differential voltage characteristic as a function of output current and temperature. Hence, it is desired that the differential input-output voltage be as small as possible. Since the series sense resistor of prior art current control circuits is connected in the series path between the input terminal and the output terminal, the sense voltage thereacross undesirably increases the input-to-output differential voltage. Thus, voltage regulators which would meet a test specification if the series sense resistor was not included therein are often rejected beacuse of the undesired differential input-to-output voltage generated by the series sense resistor.